From Wikipedia, the free encyclopedia

A secret treaty is a treaty between nations that is not revealed to other nations or
interested observers. An example would be a secret alliance between
two nations to support each other in the event of war. The opposing
nations would be unaware of the treaty and therefore unable to add
it to their calculations, which could obviously result in a
difficult situation for the party that declared war when they are
suddenly confronted with the troops of two or even three nations.
Secret treaties were common before the First World War, and many blamed them for
helping spark that conflict. Other noted secret treaties include
the sections of the Molotov-Ribbentrop Pact that divided Eastern Europe
and the Quadripartite Agreement on
intelligence matters between the United Kingdom, the United States, Canada, and Australia.